rust: add support for b_ndebug

Rust has a `debug_assert!()` macro, which is designed to be toggled on
the command line. It is on by default in debug builds, and off by
default in release builds, in cargo. This matches what meson's b_ndebug
option does in `if-release` mode.

docs/markdown/snippets/rustc-ndebug.md

@@ -0,0 +1,6 @@
+## Rust now supports the b_ndebug option
+
+Which controls the `debug_assertions` cfg, which in turn controls
+`debug_assert!()` macro. This macro is roughly equivalent to C's `assert()`, as
+it can be toggled with command line options, unlike Rust's `assert!()`, which
+cannot be turned off, and is not designed to be.

mesonbuild/compilers/rust.py

@@ -63,7 +63,7 @@ class RustCompiler(Compiler):
                          is_cross=is_cross, full_version=full_version,
                          linker=linker)
         self.exe_wrapper = exe_wrapper
-        self.base_options.add(OptionKey('b_colorout'))
+        self.base_options.update({OptionKey(o) for o in ['b_colorout', 'b_ndebug']})
         if 'link' in self.linker.id:
             self.base_options.add(OptionKey('b_vscrt'))
 
@@ -204,6 +204,10 @@ class RustCompiler(Compiler):
         # pic is on by rustc
         return []
 
+    def get_assert_args(self, disable: bool) -> T.List[str]:
+        action = "no" if disable else "yes"
+        return ['-C', f'debug-assertions={action}']
+
 
 class ClippyRustCompiler(RustCompiler):
 

test cases/rust/1 basic/meson.build

@@ -1,4 +1,4 @@
-project('rustprog', 'rust')
+project('rustprog', 'rust', default_options : ['b_ndebug=true'])
 
 e = executable('rust-program', 'prog.rs',
   rust_args : ['-C', 'lto'], # Just a test
@@ -7,3 +7,14 @@ e = executable('rust-program', 'prog.rs',
 test('rusttest', e)
 
 subdir('subdir')
+
+# this should fail due to debug_assert
+test(
+  'debug_assert_on',
+  executable(
+    'rust-program2',
+    'prog.rs',
+    override_options : ['b_ndebug=false'],
+  ),
+  should_fail : true,
+)

test cases/rust/1 basic/prog.rs

@@ -1,4 +1,5 @@
 fn main() {
     let foo = "rust compiler is working";
+    debug_assert!(false, "debug_asserts on!");
     println!("{}", foo);
 }